218 



Prof. G. Forbes. On a 



[Feb. 18, 



thermopile, to equal the sensitiveness of the latter, and a larger 

 number if it occupied a greater space. On the whole, it seems 

 probable that by specially designing a galvanometer to match the 

 line-thermopile the arrangement would be about as sensitive as the 

 new instrument in the form hitherto described, but the simplicity and 

 cheapness of construction of the latter commends it. 



The next apparatus was made according to the following instruc- 

 tions : — 



Take a wedge whose distance from the apex to the base is about 

 6 cm., the base of the triangular section of the wedge being about 

 3 cm., and the width of the wedge 6 cm. The wedge is half of 

 antimony and half of bismuth, the division being made by the 

 medial plane perpendicular to the three rectangular faces of the 

 wedge cut off the apex of the wedge by a plane parallel to the base 

 of the wedge, and exposing a surface of 1-J cm. width. This is the 

 surface which receives the radiations, and it is lamp-blacked. A hole 

 about 1 cm. diameter is now drilled (or it is better to file it out before 

 the two metals are soldered together) through the two sides of the 

 wedge, so as to leave only a thin wall along the junction of the 

 metals at the surface which receives the radiations. A Thomson cell 

 with suspended mirror and magnet is inserted in this hole and the 

 instrument is complete, and ready to be placed inside the brass box 

 with cone already described. 



The resistance of this cell is very low and its sensitiveness thereby 

 increased. Moreover this type has a great advantage in the fact that 

 the mass of metal acts as a damper upon the vibrations of the magnet, 

 and so we have a dead-beat instrument. 



The diameter of the cone to receive radiations at its mouth was 

 5 cm. A candle at a distance of 30 cm. from the mouth of the cone 

 gave a deflection of 52 divisions, a reading being easily made correct 

 to one division. A duplex lamp burning paraffin oil at a distance of 

 1^ metres gave a deflection of 60 divisions. 



The author takes this opportunity to describe a method of carrying 

 the delicate Thomson cells without danger of breaking the silk fibre 

 suspension. The cell consists externally of a brass tube. A horse- 

 shoe magnet is obtained with the distance between its legs small 

 compared with the diameter of the above-mentioned tube. The tube 

 is placed so as to rest on the inner edges of the legs of the magnet, 

 with the mirror over the poles of the magneb, the mirror magnets 

 having their poles over poles of opposite name of the horse-shoe 

 magnet, and with the silk fibre next to the magnet. The mutual 

 magnetic attraction takes the tension off the silk fibre and holds the 

 mirror fixed in position, and the fibre cannot be broken by a blow 

 given to the apparatus. 



Fig. 1 is a sketch of the first arrangement in the form of a tube. 



